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NIP-04: clarify how shared secret is computed

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Jeff Thibault 2023-02-10 08:18:40 -05:00 committed by fiatjaf
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@ -14,6 +14,8 @@ A special event with kind `4`, meaning "encrypted direct message". It is suppose
**`tags`** MAY contain an entry identifying the previous message in a conversation or a message we are explicitly replying to (such that contextual, more organized conversations may happen), in the form `["e", "<event_id>"]`.
**Note**: By default in the [libsecp256k1](https://github.com/bitcoin-core/secp256k1) ECDH implementation, the secret is the SHA256 hash of the shared point (both X and Y coorinates). In Nostr, only the X coordinate of the shared point is used as the secret and it is NOT hashed. If using libsecp256k1, a custom function that copies the X coordinate must be passed as the `hashfp` argument in `secp256k1_ecdh`. See [here](https://github.com/bitcoin-core/secp256k1/blob/master/src/modules/ecdh/main_impl.h#L29).
Code sample for generating such an event in JavaScript:
```js